The invention relates to a device for measuring the internal resistance of a linear lambda probe of an internal combustion engine.
The dynamic resistance of the diffusion barrier of a linear lambda probe which is arranged in the exhaust tract of an internal combustion engine in order to determine the fuel/air mixture fed to the internal combustion engine has a temperature dependence which leads to errors in the transmission ratio, i.e. in the measurement result. This is countered by measuring the probe temperature and adjusting it to a constant value by means of a heating element installed in the lambda probe. For reasons of cost, a separate thermal element is not used for temperature measurement here, and instead the highly temperature dependent internal resistance Ris of the lambda probe is measured.
A customary measurement method for determining the internal resistance Ris is to apply to the probe an alternating current which has been acquired by means of a square wave oscillator. An alternating voltage then drops across the internal resistance Ris. This alternating voltage is amplified and rectified and can be fed to a microprocessor for adjusting the temperature.
This measurement method results in a falsification of the output signal with a roof-shaped slope of the square wave signal (for example owing to excessively small coupling capacitors or effects of the probe control loop), and there is a high degree of sensitivity to EMC interference due to the rapid response of the rectifier.